1. Field of the Invention
The present invention relates to an atomic probe microscope for observing a fine surface configuration of a sample, using a pointed probe.
2. Description of the Related Art
Atomic probe microscopes include a scanning tunneling microscope (STM), an atomic force microscope (AFM) and a magnetic force microscope (MFM).
The STM was proposed in 1982 by Binnig, Rohrer, et al. It can observe a surface configuration of an electrically conductive sample on the atomic order. The STM is described in detail in "Surface Studies by Scanning Tunneling Microscope", G. Binnig, H. Rohrer, Ch. Gerber and E. Weibel, Physical Review Letters, Vol. 49, 57 (1982). The STM has an electrically conductive probe which is supported in the vicinity of the surface of an electrically conductive sample. The probe tip is approached to the sample surface at a distance of 1 nm. A voltage is applied across the probe and the sample, thereby causing a tunnel current to flow there-between. The tunnel current varies depending on the distance between the probe and the sample. If the distance varies by 0.1 .mu.m, the current increases about ten times or decreases to about one tenth. In the observation, the probe is moved along the sample surface (e.g. "raster scan"). While the probe is being moved, the distance between the probe tip and the sample surface is controlled using a finely movable element such as a piezoelectric element, so as to keep the intensity of the tunnel current between the probe and sample at a constant value. Thus, the distance between the probe and sample is kept constant, and the locus of the probe tip creates a curved surface that is parallel to the sample surface and representative of the surface configuration of the sample. Accordingly, a three-dimensional image representing the sample surface is formed on the basis of positional data relating to the probe tip which is calculated from the voltage applied to the piezoelectric element.
On the other hand, the atomic force microscope (AFM) is proposed as an apparatus capable of observing the surface configuration of an insulative sample in the atomic order. It is described in detail in "Atomic Force Microscope", G. Binnig, C. F. Quate, Physical Review Letters, Vol. 56, 930 (1986). In the AFM, the probe is supported by a soft cantilever. When the probe is moved close to the sample surface, a van der Waals attractive force acts between an atom at the tip of the probe and an atom on the sample surface. Then, if both atoms move close to each other so as to nearly contact, a repulsive force occurs therebetween due to the Pauli exclusion principle. The attractive force and repulsive force (between atoms) are very weak and about 10.sup.-7 to 10.sup.-12 [N]. In general, when observation is effected with an atomic force microscope, the probe can approach the sample surface to such a distance that the cantilever is somewhat displaced owing to the inter-atomic force exerted on the atom at the probe tip. If the probe is scanned along the sample surface from this state, the distance between the probe and the sample varies in accordance with the configuration of the sample surface and, accordingly, the amount of displacement of the cantilever varies. The variation in displacement of the cantilever is detected. A detection signal obtained by this detection is used as a configuration signal. Alternatively, the displacement of the cantilever is subjected to feedback control on the basis of the detection signal by use of a fine movement element, such as a piezoelectric element, so as to keep the displacement of the cantilever at a constant value, and a control signal obtained by this feedback control is used as a configuration signal. As a result, the probe tip moves while describing a curved plane in parallel to the sample surface. An image of the surface configuration of the sample is constructed on the basis of the configuration signal in synchronism with the scan of the probe.
The MFM (magnetic force microscope) has a probe made of a magnetic material. The other structural features of the MFM are basically identical to those of the atomic force microscope (AFM). Like the AFM, the probe of the MFM is scanned along the sample surface while a magnetic force acting between magnetic particles of the probe and the sample is kept contact, thereby obtaining an image of the surface configuration of the sample.
The cantilever employed in the AFM or MFM should desirably be formed in an elongated shape of a material having a light weight and a high elastic coefficient, since the cantilever needs to be displaced with high responsiveness to a weak force (inter-atomic force or magnetic force). However, if the length of the cantilever increases, the characteristic frequency decreases. As a result, the responsiveness to the surface configuration of the sample at the scan time is degraded, and the removal of external vibration noise becomes difficult. Generally, the length of the cantilever is limited to 1000 .mu.m or less and the characteristic frequency is set to about 10 to 100 kHz. Thus, the amount of displacement of the cantilever is limited, and high sensitivity to the displacement is required.
According to a method of detecting displacement of such a cantilever, an STM is constituted on the rear face (the face on which the probe is not provided) of the cantilever, and displacement of the cantilever is detected as a variation of tunnel current. In this case, if the cantilever is electrically conductive, no special treatment is required; however, if it is electrically insulative, the surface of the cantilever is coated with an electrically conductive material, for example, by means of deposition. The STM has sufficient sensitivity to the distance between the probe and the cantilever. However, since an inter-atomic force acts between the probe and the cantilever, exact measurement cannot be carried out.
According to another method, an optical reflecting surface is provided on the rear face of the end portion of the cantilever, a beam from a semiconductor laser, a ruby solid laser or an argon gas laser is made incident on the reflecting surface, and a reflection angle varying in accordance with displacement of the cantilever is detected by a PSD (position signal detector). In this method, however, if the incidence angle of the beam is increased to enhance sensitivity, the size of the apparatus increases. Consequently, the characteristic frequency decreases and the sensitivity decreases. In addition, the beam incident on the cantilever surface has a width, and, in order to enhance the resolution of the reflection angle, the flatness of the reflecting surface must be improved. This is not easy, however.
According to still another method, the light emitted from the laser is divided into a reference beam and a detection beam. The detection beam is radiated on the optical reflection surface formed on the rear of the end portion of the cantilever. A reflected beam from the reflection surface is caused to interfere with the reference beam, and an interference output is photoelectrically detected. In order to obtain good sensitivity, the light path length of the reference beam system must be equalized to that of the detection beam system, so as to cancel ambient influence (variation in temperature, atmospheric pressure, etc.). This makes the apparatus complex. If the reference beam system and the detection beam system are formed separately, it is difficult to equalize the characteristic frequencies of the respective light paths. Thus, the sensitivity is deteriorated owing to ambient influence.
According the atomic probe microscope, the probe is moved relative to the sample surface, in order to measure the sample surface configuration. During the movement, the probe is servo-controlled in the z-direction vertical to the sample surface, so as to keep the distance between the probe and the sample constant.
In the STM, the servo control of the probe is carried out by feed-back controlling the z-axial position of the probe so as to keep constant the tunnel current flowing between the probe and the conductive sample. Thus, if dust is on the sample surface or part of the sample surface is coated with an oxide film, the probe approaches the sample while removing dust or oxide film.
In either the AFM or the STM, the probe moves along the sample surface. If oxide film or dust exists above the tip of the probe, the probe suffers a shearing force by the oxide film or dust in the x- or y-direction. Consequently, there occurs an error between the actual position of the probe tip and the position found on the basis of the voltage applied to the piezoelectric element for finely moving the probe.